One type of subsea well has a wellhead housing at the sea floor. One or more casing hangers land in the bore of the wellhead housing, each secured to an upper end of a string of casing. After cementing the casing, a running tool sets a packoff or annulus seal between the casing hanger and the wellhead housing.
One type of packoff has a metal ring with inner and outer legs separated by an annular slot. The running tool pushes an energizing ring into the slot, which radially deforms the inner and outer legs into sealing engagement with the wellhead housing and the casing hanger.
One or both of the seal surfaces in the wellhead housing and on the casing hanger may have a set of wickers. The wickers are parallel grooves, each having a sharp crest. The sharp crests of the wickers embed into the seal ring surface when set with the energizing ring. The embedded wickers form seals as well as resisting axial movement of the seal assembly relative to the wellhead and casing hanger.
The wellhead housing will be filled with a liquid, which may be drilling fluid, before the seal ring sets. When the seal ring wall moves radially into engagement with the wickers, some of the liquid may be trapped in the wickers, forming a hydraulic lock. The trapped liquid can result in high pressure build up in the wickers, which restricts the depth that the wickers embed into the seal ring. The lesser depth or engagement can reduce the effectiveness of the seal ring.